Flat Panel Liquid Crystal Displays typically include a light source illuminating the Liquid Crystal Display. In many instances, and particularly where Liquid Crystal Displays are used in a cockpit of an aircraft, the display must be viewed under very high ambient light levels which may be as high as 10,000 ft-lamberts. In order to be visible at these ambient brightness levels the brightness level of the display must also be very high -2000 ft lamberts or more. Fluorescent lamps are very useful for this application as their efficiency as well as their output is very high and can easily reach 2000 to 4000 ft lamberts. Fluorescent lamps, typically include a filler gas mixture of mercury vapor and a rare gas such as argon. The mercury vapor emits ultra violet radiation which excites the phosphor deposited on the inner surface of the lamp wall to produce light in the spectrum.
Fluorescent lamps are, however, temperature sensitive and the lamp filler gas and the phosphor operates at optimum efficiency with a wall temperature between 40.degree. and 50.degree. C. (i.e., 104.degree. to 122.degree. F.). The light output drops off rapidly outside of this optimum temperature range and at very low temperatures (i.e., -30.degree.-50.degree. C.), fluorescent lamps are essentially inoperative. Liquid Crystal Displays when utilized in aircraft applications, may often be subjected to extremely low ambient temperature conditions. For example, it is not uncommon in aircraft applications (where aircraft may be parked overnight in extreme cold) for the ambient temperature to be as low as -20.degree. or -30.degree. C. Operation at these extremely low temperatures introduces problems for fluorescent light sources which are otherwise prime candidates because of their high brightness and their high efficiencies.
In the past, it has been proposed to use heating elements wound around the fluorescent lamp to maintain the lamp wall temperature at the desired level for maximum brightness. In such a system a temperature sensor located in the vicinity of the lamp controls a power supply which drives current through the heaters to maintain the wall temperature of the fluorescent lamps at the desired level. However, the use of heaters, sensors, signal processors and power supplies add cost, and complexity, to the system and require additional space. A need exists for a different and more effective way of maintaining the wall temperature, and hence the light output, of a fluorescent lamp at the desired optimal level even at low ambient temperatures.
In a fluorescent lamp approximately 21% of the input energy is converted into visible light either directly or through the U.V. conversion processes and the remaining 79% is converted to heat; 42% is dissipated directly and 37% is dissipated in the form of infrared radiation.
Applicant has discovered a system in which a portion of the heat dissipated by the lamp is utilized to maintain the lamp wall temperature at the desired level. This is achieved by providing a jacket around the lamp which contains a fluid (or mixture of fluids), which is selectively evaporated and condensed to maintain the lamp wall temperature in the optimum temperature range. At low temperatures fluid in the vapor phase condenses to form a partial vacuum in the jacket around the lamp. The vacuum minimizes convectional, and conductive heat losses from the lamp and raises the temperature of the wall toward the optimal conditions even though the ambient temperature outside of the jacket is much lower.
As the wall temperature rises above the desired optimal level, the fluid (or mixture of fluids) in the liquid phase is vaporized to extract heat from the wall, both by means of the latent heat of vaporization of the fluid as it changes from the liquid to the vapor phase and then through heat transfer through the vapor in the jacket. Thus, above the critical temperature range heat is removed from the fluorescent lamp wall to the vapor and thence to the outer wall of the jacket. At very low temperatures all of the fluid condenses to the liquid phase forming a vacuum.
Suitable mixtures of material such as bromine, alcohol, water, etc. may be used in suitable proportions to establish the desired transition or vaporization temperature at which the fluid vaporizes to remove heat and maintain the wall temperature.